It has been noted that obtaining a high quality stack depends in large part on the constancy of the stacking pressure. However, this constant stacking pressure for a given signature of a given production changes from one production to another because of the change in conditions of production, given the variations of the parameters of production which are the thickness of the paper, the thickness of the ink, the variation of weight, of the interval of the lap, etc.
It has also been noted that the production conditions can also change within a same production. As a result, it is necessary to be able to adapt the stacking pressure between two different productions but also within a same production so as to achieve a stack of good quality. It is thus necessary to be able to measure continuously this stacking pressure so as to adjust it.
Most of the stacking devices do not comprise a device for the regulation of the stacking pressure and the latter is thus affixed indirectly by the value of the speed of advance of the stacking table. In this type of "stolen" device, it is not possible to intervene so as to permit adjustment of the stacking pressure in the course of production. Only a variation of the speed of the stacking table permits adjusting, as needed, the stacking pressure between different productions.
A solution seeking to permit adjustment of the stacking pressure consists in using in the stacking device a floating stacking head as in EP-A-0 623 542, which is to say that there can be obtained a variable stacking point permitting variation of the stacking pressure. However, in this case, the detection device operating by gravity on the floating stacking device must overcome its own inertia. Moreover, the stacking can be disturbed because the stacking pressure must be sufficiently great to overcome the inertia of the floating device, which risks spoiling the signatures by marking on the fold (unrolling phenomenon) or on the printed matter (staining phenomenon).
The stacking pressure depends on the parameters of arriving and leaving of the signatures during stacking. Thus, the larger the arrival flow rate of the signatures for a constant departure rate of the signatures, the greater the stacking pressure, and vice versa. On the other hand, for a constant flow rate of arrival of signatures, the greater the exit flow rate of signatures the less the stacking pressure, and vice versa.
It is therefore important that the measurement of the stacking pressure in the course of a production permits modifying certain parameters of stacking, in particularly the output flow rate of the signatures, so as to obtain a constant stacking pressure whilst preserving stacking conditions which must not be upset by this measurement. Thus, it is important to maintain a stacking point as stationary as possible.
One solution thus consists in providing a device for regulating the stacking pressure in which this pressure is measured. Thus, there can be positioned a plate in the vertical arrival path of the signatures on the stacking table, as in EP-A-0 339 002. This plate is pivotally mounted at its upper end and is connected to a lever connected at its lower end to a spring. Upon arrival of the signatures on the table, the plate pivots as a function of the pressure of said signatures and the lever also moves. A sensor detects the movement of said signature and controls the adjustment of the speed of advance of the table as a function of the information received.
However, in such a device, the plate interferes with the arrival of the signatures or rubs against said signatures, which upsets the stacking which thus is not sufficiently precise.
In JP-A-59102761, it is proposed to position a pressure detector in the vertical arrival path of the signatures, which drives the advance of the stacking table by a certain value when it detects a value of pressure above a predetermined level. Such a device does not permit continuous regulation of the stacking pressure and is moreover adapted to adjust, stepwise, the advance of the signatures on the stacking table. Because of this, there is obtained a stacking whose precision is only so-so and is disturbed.
U.S. Pat. No. 4,772,003 discloses a device in which a vertical supply belt for the signatures is swingably mounted so as to constitute a pressure sensor, the swinging being measured to adjust as a result the speed of advance of the stack. However, such a device must first overcome its own inertia before being able to give an accurate measurement of the stacking pressure. Because of this, the measurement is often imprecise or of threshold effect.